Prostaglandins (hereinafter, referred to as PGs) are members of class of organic carboxylic acids, which are contained in tissues or organs of human and other mammals, and exhibit a wide range of physiological activities. PGs found in nature (primary PGs) have, as a general structural property thereof, a prostanoic acid skeleton as shown in the formula (A):

On the other hand, some synthetic analogues have modified skeletons. The primary PGs are classified into PGAs, PGBs, PGCs, PGDs, PGEs, PGFs, PGGs, PGHs, PGIs and PGJs on the basis of the structural property of the five membered ring moiety, and further classified into the following three types by the number and position of the unsaturated bond in the carbon chain moiety.    Type 1 (subscript 1): 13,14-unsaturated-15-OH    Type 2 (subscript 2): 5,6- and 13,14-diunsaturated-15-OH    Type 3 (subscript 3): 5,6-, 13,14-, and 17,18-triunsaturated-15-OH.
Further, PGFs are classified on the basis of the configuration of the hydroxyl group at the 9-position into α type (wherein the hydroxyl group is of the α-configuration) and β type (wherein the hydroxyl group is of the β-configuration).
In addition, some 15-keto-PGs (PGs having an oxo group at position 15 in place of the hydroxy group) and 13,14-dihydro-15-keto-PGs have been known as substances naturally produced by enzymatic actions during metabolism of the primary PGs. 15-keto-PGs have been disclosed in U.S. Pat. Nos. 5,073,569, 5,534,547, 5,225,439, 5,166,174, 5,428,062 5,380,709 5,856,034 6,265,440, 5,106,869, 5,221,763, 5,591,887, 5,770,759 and 5,739,161. The contents of these publications are herein incorporated by reference.
Some prostaglandin compounds have been known to be useful as pharmaceutical agents in the ophthalmic area, namely as an ocular hypotensive agent or an agent for treatment of glaucoma. For example, latanoprost, that is 13,14-dihydro-17-phenyl-18,19,20-trior-PGF2α isopropyl ester, travoprost, that is 16-(3-trifluoromethyl phenoxy)-17,18,19,20-tetranor PGF2α isopropyl ester and bimatoprost, that is 17-phenyl-18,19,20-trinor-PGF2α N-ethylamide have already been placed on the market under the name of Xalatan™, Travatan™ and Lumigan™ eye drops for the treatment of glaucoma and ocular hypertension.
Further, 15-keto-prostaglandin compounds have been known to be useful as pharmaceutical agents in the ophthalmic area, namely as an ocular hypotensive agent or an agent for treatment of glaucoma, see U.S. Pat. Nos. 5,001,153; 5,151,444, 5,166,178, 5,194,429 and 5,236,907, for treatment of cataract, see U.S. Pat. Nos. 5,212,324 and 5,686,487, for increasing the choroidal blood flow, see U.S. Pat. No. 5,221,690 and for treatment of optic nerve disorders, see U.S. Pat. No. 5,773,471. The contents of these USPs are herein incorporated by reference. Especially, isopropyl ester of 13,14-dihydro-15-keto-20-ethyl-PGF2α has already been placed on the market under the name of Rescula™ (common name: isopropyl unoprostone) ophthalmic solution for treatment of glaucoma and ocular hypertension. Rescula™ ophthalmic solution provides sufficient ocular hypotensive effect by administrating twice a day.